火山岩貯留岩の岩相と孔隙

特に南長岡ガス田における流紋岩について

抄録

Several large hydrocarbon deposits in deep-seated volcanic-rocks in "Green Tuff" horizon have been discovered around Kashiwazaki to Nagaoka cities in Niigata prefecture. This paper studies the properties of porosity and the distribution of rock facies in the volcanic-rock reservoirs.
In the Minami Nagaoka gas field, rhyolite reservoirs, having total thickness of 1, 000m and gas column of more than 800m, occur at a depth of 3, 800m and deeper below the surface. The rhyolite is divided into lava, pillow breccia and hyaloclastite in terms of rock facies which resulted from drastic chilling and shattering in submarine volcanism. Pore spaces in the rhyolite are classified into several porosity types in terms of their origin, shape and size. The properties of porosity is closely related to each rock facies. The favourable rock facies as reservoirs are pillow breccia and lava facies. Meso- to micro-sized vugs play an important role in total porosity, while micro-sized secondary fractures are effective to permeability. In order to predict productive zones, it is important to understand the relavant volcanic-rock facies.